This invention relates to the suppression of noise emanating from a jet engine. More particularly this invention relates to the suppression of noise propagating forward through the air inlet duct of a jet engine.
The suppression of noise radiated by jet engines is a problem that has been receiving increasing attention, particularly within the past few decades. In this regard, noise limitations imposed by regulations and the like have often precluded operation at the most desirable power levels and have even curtailed aircraft operation in and around airports located in or near residential areas. One source of jet engine noise, commonly denoted as inlet noise, is due to sound waves propagating in the forward direction through the inlet duct of a jet engine. Inlet noise is due to a number of sources, such as high speed fan rotation in a turbofan engine and compressor operation.
Several prior art attempts have been made to reduce inlet noise. In the main, the prior art has concentrated on mounting sound-absorbing material on the air inlet duct wall or on the air inlet centerbody. Because of the limited duct wall area within most conventional jet engines, such prior art attempts have often required the addition of structural members such as annular rings or vanes supported in spaced relationship with the inlet duct wall to increase the surface area upon which acoustically absorbent material can be applied or mounted--see "Progress of NASA Research Relating to Noise Alleviation of Large Subsonic Jet Aircraft," NASA report SP-189, 1968, paper #9, "Design Concepts" by Robert E. Pendley.
Another prior art attempt to eliminate or reduce air inlet duct noise is discussed in U.S. Pat. No. 3,692,141 issued to Labussiere et al. Labussiere discloses an inlet duct that includes an acoustically reflective centerbody. The centerbody is shaped to reflect inlet noise toward localized areas of the outer duct wall that are treated with acoustically absorbent linings.
The prior art, although successful to a certain extent, has failed to suppress inlet noise to the desired level. Further the prior art, because of added structural elements such as concentric rings mounted in the interior of the inlet duct, has often imposed unacceptable performance penalties due to added weight and/or restricted engine airflow. For example, the air inlet disclosed by Laboussiere et al not only imposes a weight penalty, but the bulbous centerbody necessary to form a parabolic acoustic reflector tends to impede efficient airflow through the engine inlet duct. Further, since the reflection angles appear to be strong functions of the fluid media, e.g. the density and velocity of the air flowing through the inlet duct, the noise suppression efficiency of this device will vary widely with certain engine parameters, such as turbine speed.
Accordingly it is an object of this invention to provide an inlet duct for a jet engine that significantly suppresses inlet noise without creating any undue detrimental effect on the operation of the engine.
Further, it is an object of this invention to realize jet engine inlet duct structure which is configured to reduce inlet noise without requiring the addition of noise-reducing elements.